"For reasons that are not immediately clear to me, it seems that a lot of developers who attended Microsoft's recent PDC event were surprised to hear that the company now sees HTML5 as the way forward for developing rich Internet applications - and not, as they had been expecting, Silverlight. Their surprise surprises me, because past statements by the company had already made this repositioning obvious, though perhaps not explicit."

The .NET Framework situation is harder than Silverlight. If someone uses a .NET 4.0 feature on any OS where .NET4 is not a given, then the barrier to entry is suddenly raised substantially, as deploying .NET is often an annoying endeavor.

On the other hand, Silverlight 4 is a 5mb runtime with a 90 second install time, where you dont even need to restart your browser or PC.